A properly functioning aneurysm clip is of paramount importance in the surgical ablation of intracranial aneurysms. An appropriate match between the closing force needed to ablate an aneurysm and the force exerted by the aneurysm clip must be achieved. In this study, the opening and closing forces exerted by several commonly used aneurysm clips were tested. There was a strong correlation between the classification of the clips, according to the Dujovny system, and their mechanical behavior. Minimal variability among different lots of the same clip type and minimal hysteresis were generally exhibited by the alpha mobile fulcrum class clips, such as the Yasargil, Vari-Angle McFadden, Scoville, and Mayfield clips. Significant exceptions included several types of Mayfield and Drake clips. In contrast, pivot class clips generally showed twice as much lot variability among different lots, as well as a significant amount of hysteresis. Clips of this class include the Vari-Angle, Heifetz, and Pivot clips. Because significant variations in clip force exist and because several other factors can compound these differences, it is suggested that aneurysm clips be individually tested for their closing forces before they are used in an operation.